The present invention relates to a system for determining the efficacy of a sterilization cycle. More specifically, the present invention relates to a system for reading fluorescence from a biological sterilization indicator in order to determine the efficacy of the sterilization cycle.
The sterilization of equipment and devices is critical in some industries. For example, hospitals and other medical institutions must commonly and frequently sterilize equipment and devices used in treating patients. The particular type of sterilization cycle used to sterilize such equipment can vary based on the particular equipment or devices being sterilized and based on the particular preference of the entity performing the sterilization cycle. However, all such sterilization cycles or processes are typically designed to kill living organisms which might otherwise contaminate the equipment or devices being sterilized.
Various sterilization cycles use different methods or techniques for sterilization. For instance, such sterilization cycles may include the administration of steam, dry heat, chemicals, or radiation, to the equipment or devices being sterilized. Steam sterilization is typically efficacious when the equipment being sterilized are exposed to steam having a temperature in a range of 121-132.degree. C. The equipment being sterilized are preferably exposed to the steam sterilization for approximately three minutes at 132.degree. C., and ranging to 30 minutes at 121.degree. C. One form of chemical sterilization involves exposing the devices to be sterilized to ethylene oxide gas. The devices being sterilized are exposed to the ethylene oxide gas for approximately one hour at 65.degree. C. to approximately four hours at 30.degree. C. Dry heat sterilization typically involves exposing the devices being sterilized to temperatures in a range of approximately 180.degree. C., or higher, for at least two hours. In many environments, the efficacy of the sterilization cycle is critical. Therefore, sterilization indicators are used to determine the efficacy of the sterilization cycle.
Sterilization indicators have taken a number of forms in the past. For example, biological indicators and chemical indicators are well known in the art. In conventional biological indicators, a test organism which is many times more resistant to the sterilization process than most organisms which would be present by natural contamination, is coated on a carrier and placed in a sterilizer along with the articles to be sterilized. Thus, the sterilization indicator is exposed to the same sterilization cycle as the devices being sterilized. After completion of the sterilization cycle, the carrier is incubated in nutrient medium to determine whether any of the test organisms survived the sterilization procedure. In the conventional biological sterilization indicators, growth of a detectable number of organisms normally takes at least approximately 24 hours.
The biological sterilization indicator is then examined to determine whether such growth has taken place. If so, such growth indicates that the sterilization cycle has not been efficacious, and it can be assumed that the devices which were subject to the sterilization cycle are not sterile.
Commercially available chemical indicators utilize chemicals which indicate sterility by color changes, or change from a solid to liquid state. One advantage to such chemical indicators is that the results are known by the end of the sterilization cycle. However, the results only indicate, for example, that a particular temperature has been reached for a certain period of time, or that ethylene oxide gas was present, during the sterilization cycle. Arguably, chemical indicators do not necessarily indicate whether conditions necessary for eliminating the organisms of interest have been achieved. Thus, the industry has shown a preference for biological indicators that use living organisms.
Sterilization indicator technology is disclosed in Matner et al. (U.S. Pat. Nos. 5,073,488; 5,252,484 and 5,418,167) and Foltz et al. U.S. Pat. No. 5,223,401 (the entire contents of each of which are herein expressly incorporated by reference). Those patents describe sterilization indicators having enzymes which drastically reduce the time required to indicate the effectiveness of the sterilization cycle.
Commercial examples of biological indicators with rapid response times include the 3M.TM. Attest.TM. Rapid Readout Biological Indicators available from 3M of St. Paul, Minn. (USA). The Attest Rapid Readout Biological Indicators include a flexible polypropylene vial with a spore strip which has a viable population of Bacillus Stearothermophilus spores. The vial also contains a growth medium which is a modified tryptic soy broth contained in a crushable glass ampule. With the Attest Rapid Readout Indicator designed for use with a steam sterilization cycle, the presence of a spore-associated enzyme indicates spore growth in the biological indicator. The presence of the active enzyme is measured by using a nonfluorescent substrate. The non-fluorescent substrate is converted by the active spore-associated enzyme to a fluorescent product.
If the sterilization cycle is not efficacious, both the spore and the enzyme remain active. The enzyme converts the substrate to a fluorescent product. Therefore, the fluorescence in the vial is detected, after an incubation period, to determine the efficacy of the sterilization cycle.
While 3M.TM. Attest.TM. Rapid Readout Biological Indicators significantly advanced the art, there is room for improvement. In order to detect spore growth activity through fluorescence, the Rapid Readout Biological Indicators for steam sterilization cycles typically incubate for one hour. After the incubation time, the vial is placed in a reading apparatus and the fluorescence within the vial is read. The incubation time required to obtain an accurate reading in such a prior reading apparatus could be shortened.
The prior reading apparatus associated with 3M.TM. Attest.TM. Rapid Readout Biological Indicators should be calibrated after a predetermined time out period (such as 12 hours of operation) or whenever power is interrupted to the unit. The calibration process has multiple operator steps. Further, the calibration is performed using a biological indicator which has been subjected to the sterilization cycle, and which is assumed to be sterile. Arguably, the calibration is essentially performed against an unknown value.